In a typical Class H amplifier system 100, a charge pump 104, that can produce either +/− charge pump voltage VCP (“+/−VCP”) or +/−½ charge pump voltage VCP (“+/−VCP/2”) or +/−⅓ charge pump voltage VCP (“+/−VCP/3”) or +/−1.33 charge pump voltage VCP (“+/− 4/3*VCP”) or some other voltage level, is used to power a headphone amplifier. FIG. 1 shows a conventional class H amplifier system 100 with a charge pump 104 and headphone amplifier 106 driving a headphone load 110. Class H control block 108 controls charge pump 104 and headphone amplifier 106 for driving the headphone load as shown in FIG. 1.
The charge pump 104 consists of a network of switches (not shown) controlled by a clock and generates a positive supply voltage (VDDCP) and a negative supply voltage (VSSCP), which are used as supplies for the ground centered headphone amplifier. The class H control block 108 determines the mode of operation for charge pump 104 based on the magnitude of an audio signal provided to the headphone load 110. One supply voltage, VCP, for the charge pump 104 in mobile applications is 1.8 V, and different modes of the charge pump 104: +/−VCP (1.8 V mode) is shown as “mode 1,”+/−VCP/2 (0.9 V mode) is shown as “mode 2,”+/−VCP/3 (0.6 V mode) is shown as “mode 3,”+/−4*VCP/3 (2.4 V mode) is “mode 4.” Each mode of the charge pump 104 may involve setting a different combination of the network of switches to be on or off. External pumping fly capacitors 112 (Cfly1) and 114 (Cfly2), which are external to the charge pump 104, may be 2.2 uF. External hold capacitors 116 (Chold1) and 118 (Chold2) may be 2.2 uF.
An external power management integrated circuit (PMIC) 102 generates the charge pump voltage VCP, such as from a supply voltage VP of a battery (not shown), and may be an inductor-based buck converter or low dropout regulator. In steady state operation, the charges on capacitors 112, 114, 116, and 118 settle to their respective designed value. For example, when the charge pump operates in mode 1 (1.8V mode) at steady state, the charge held by capacitor 112 is 0, while the charges for capacitors 116, 114, and 118 is each 1.8*2.2 uF. However, when the charge pump 104 operates in mode 2 (0.9V mode) at steady state, the charges on all four capacitors 112, 114, 116, and 118 is each 0.9*2.2 uF. The charges held by each of the four capacitors 112, 114, 116, and 118 is 0.6*2.2 uF in mode 3. In mode 4, the charge held by capacitor 112 is 0, while the charges for capacitors 114, 116, and 118 is each 2.4*2.2 uF. The charge difference between modes depends on the charge pump 104 configuration before and after the mode transition.
During charge pump mode transitions, the difference in charge on the capacitors 112, 114, 116, and 118 is provided by the current from power supply voltage VP or VCP. As shown in FIG. 2, for a mode transition from a lower to higher supply, such as from 0.6 V to 0.9 V, 0.9 V to 1.8 V, or 1.8 V to 2.4 V, a final mode has a higher amount of charge on the capacitors 112, 114, 116, and 118 than a starting mode. In any of these positive voltage slope transitions (with respect to voltage VDDCP), instantaneous current is drawn from the voltage supply VCP/VP to charge the capacitors 112, 114, 116, and 118. For a mode transition from higher to lower supply, such as from 2.4 V to 1.8 V, 1.8 V to 0.9 V, or 0.8 V to 0.6 V, a final mode has a lower amount of charge on the capacitors 112, 114, 116, and 118 than a starting mode. In any of the negative voltage slope transitions (with respect to voltage VDDCP), excess charge will push current to the voltage supply VCP/VP, if the charge is not dissipated internally in the charge pump 104 or other component of an integrated circuit. In both positive and negative voltage slope transitions, the current that is drawn from or pushed to the supply is abrupt and large (in the order of hundreds of milli-amps), because such transitions involve charging/discharging relatively large external capacitors. When the PMIC 102 cannot react to the sudden change in current, the supply voltage may also change abruptly, causing potential reliability concerns.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved amplifier systems, particularly for consumer-level devices. Embodiments described here address certain shortcomings but not necessarily each and every one described here or known in the art.